Although the agents and the mechanisms that cause breast cancer are still uncertain, there is substantial evidence that breast cancer risk is strongly dependent on endocrinological and reproductive conditions, such as early menarche, late menopause, and nulliparity. An early first full term pregnancy, on he other hand, confers a lifetime protection. Experimentally we have demonstrated that the protection conferred by pregnancy is mediated by induction of breast differentiation, which is manifested through obular development, reduced cell proliferation and steroid hormone receptor content, and a "genomic signature" that is associated with refractoriness to chemically induced carcinogenesis. This application proposes to test the hypothesis that early pregnancy imprints in the breast permanent genomic changes that reduce the lifetime risk of the developing breast cancer. This hypothesis will be demonstrated through the following specific aims: 1: To characterize the specific gene expression profile of women at "low" and 'high" risk of developing breast cancer because of reproductive history. Gene expression in normal breast tissue obtained from postmenopausal women with a history of one or more early full term pregnancies will he compared to gene expression in normal breast tissue from postmenopausal women who are nulliparous. For this purpose we will perform a large-scale analysis of gene expression using available human cDNA libraries for determining the expression pattern of known and unknown (ESTs) genes. 2: To determine if gene clusters differentially expressed in women at risk of developing breast cancer because of reproductive history (identified in Specific aim 1) are differentially expressed in the breast issue of postmenopausal women with breast cancer compared to postmenopausal women who do not have breast cancer. For this purpose we will perform a case-control study to compare gene expression profiles in the normal, breast tissue of postmenopausal women with invasive breast cancer vs. postmenopausal women with benign breast disease. Knowledge gained through these studies will allow us to identify the "genomic signature" induced by an early pregnancy, and that if proven to be functionally relevant would serve as molecular markers for assessing breast cancer risk in large populations.